In graphic detail, Sharon Bialek, a registered Republican from Chicago, told a press conference in New York that Mr Cain had put his hand up her skirt and pulled her head towards his lap after she approached him for help in finding a new job.

Ms Bialek is the fourth woman to accuse Mr Cain of sexual harassment in the past 10 days, but last night she became the first to be speak publicly about the conduct of the former president of America's National Restaurant Association (NRA).

The 50-year-old, who once hosted a television cookery show, recounted how she had met Mr Cain in Washington DC shortly after she had been sacked by the association's educational foundation in July 1997. After dinner, Mr Cain drove her toward the association's offices and then parked nearby.

"Instead of going into the offices, he suddenly reached over and put his hand on my leg under my skirt and reached for my genitals. He pushed my head towards his crotch.

"I said, 'What are you doing? You know I have a boyfriend. This isn't what I came here for.' Mr Cain said, 'You want a job, right?' I asked him to stop, which he did. I asked him to take me back to my hotel, which he did."

The pair had earlier sat next to each other three times at meals during an association conference in Chicago. Ms Bialek said that she had subsequently contacted him at the suggestion of her then boyfriend because of her employment difficulties.

Appearing at the press conference alongside the celebrity lawyer Gloria Allred, Ms Bialek, the single mother of a 13-year-old boy, was nervous but composed as she became the first public accuser after sexual harassment allegations from three other anonymous women had swirled around Mr Cain for a week.

The political career of the former pizza company executive, who is the surprise leader in Republican polls, was hanging in the balance after she spoke. Reporters were handed affidavits from her ex-boyfriend and another man she was said to have told about the alleged assault. Criminal charges are not possible because the statute of limitations has expired.

Tactic

Mr Cain's tactic of refusing to answer questions about harassment allegations now appears untenable. Until Miss Bialek's appearance, he appeared to have weathered the storm with polls indicating his support was holding up.

Ms Allred said Ms Bialek had "reached out to Mr Cain for help finding another job". "Instead, Mr Cain decided to provide her with his idea of a stimulus package," said Ms Allred, to a chorus of groans from the crowd.

Ms Bialek said she was coming forward "to give a face and a voice" to all women who allege Mr Cain harassed them: "I want you, Mr Cain, to come clean. Admit you were inappropriate to people, and then move forward Mr Cain."